1. Field of the Invention
The present invention relates to semiconductors and semiconductor fabrication, and more particularly, to the fabrication of diodes.
2. Background
The downscaling of semiconductor processes to deep submicron dimensions has permitted the layout of very dense ESD protection diodes which are required to support high current densities under an ESD discharge. Thus, a conventional diode for ESD protection is capable of providing a very high conductivity per unit area. However, the current failure point, which defines the limitation on the maximum current through a diode is typically very low for a conventional diode fabricated with deep submicron technology. In typical instances, the current failure point of a conventional diode for ESD protection is determined by metal temperature effects to a greater extent than diode contact effects and silicon temperature effects.
In a conventional diode for ESD protection, a plurality of elongate cathode metal stripes and a plurality of elongate anode metal stripes each having a predetermined constant width are implemented on a semiconductor substrate. The cathode metal stripes are connected to a cathode feeder bus while the anode metal stripes are connected to an anode feeder bus. The cathode and anode metal stripes are alternately spaced from each other on the semiconductor substrate, which typically comprises silicon. A plurality of diffusion windows are provided along each of the anode and cathode metal stripes to serve as contacts for P and N junctions, respectively.
During the fabrication of a conventional diode for ESD protection, a P+ dopant is implanted through field oxide windows along the anode metal stripes into regions of the semiconductor substrate underneath the anode metal stripes to form respective P+ regions. In a similar manner, an N+ dopant is implanted through field oxide windows along the cathode metal stripes into regions of the semiconductor substrate underneath the cathode metal stripes to form respective N+ regions. In a typical fabrication process, anode and cathode metal stripes of a constant width are implemented in a conventional diode for ESD protection with a relatively simple layout geometry while providing a relatively high conductivity per unit area along the cathode and anode metal stripes.
The current failure point of a conventional diode for ESD protection is primarily determined by the current per unit of metal width of each of the anode and cathode metal stripes because the heating of the metal stripes is essentially adiabatic. In a conventional diode for ESD protection which includes anode metal stripes of a constant width, the current flow starts at zero at the end of each of the anode metal stripes and ramps up in an essentially linear manner until a peak current is reached at the feeder bus tie point, which is the location at which the anode metal stripe is connected to the anode metal feeder bus. A current failure typically occurs at one or more locations at which the metal stripes are connected to the respective anode and cathode metal feeder busses, because current densities at these locations are at a maximum in a conventional diode for ESD protection.
The present invention provides a diode for ESD protection, roughly comprising:
a cathode feed bus;
an anode feeder bus positioned opposite the cathode feeder bus;
a plurality of elongate cathode conductor stripes each having first and second end portions of different widths, the first end portion connected to the cathode feeder bus, the width of the second end portion narrower than that of the first end portion; and
a plurality of elongate anode conductor stripes each having first and second end portions of different widths, the first end portion connected to the anode feeder bus, the width of the second end portion narrower than that of the first end portion, the cathode conductor stripes and the anode conductor stripes alternately spaced from each other.
Advantageously, the diode in an embodiment according to the present invention is capable of providing improved electrostatic discharge (ESD) protection against current failure by reducing current densities along the anode and cathode conductor stripes, especially at or near the locations at which the anode and cathode conductor stripes are connected to the anode and cathode feeder busses, respectively. Furthermore, improved protection against current failure can be achieved by the diode in an embodiment according to the present invention using existing deep submicron fabrication technology without the need for more complicated fabrication processes.